Retinal photoreceptor cells are only one photosensor in a mammal, and has previously been intensively studied physiologically, biochemically, anatomically and clinically (JP-A-2002-325571). However, mechanism of development and differentiation into retinal photoreceptor cells has been unknown at all. As a causative locus for human genetic retinal degeneration which is a disease resulting from abnormalities of retinal photoreceptor cells, at least 145 loci are known, but an established method of treating the disease has not been present yet, and patients are suffering from severe eyesight disorder. Therefore, elucidation of causes, and establishment of therapeutic method for retinal degeneration leading to loss of eyesight or severe eyesight disorder have been desired. In addition, since degeneration or dyscrasia of retinal photoreceptor cells is seen in many retinal diseases such as not only retinitis pigmentosa, but also diabetic retinopathy and macular degeneration, it is very important to elucidate molecular mechanism of development and differentiation of retinal photoreceptor cells, in order to enable degeneration or neogenesis of retinal photoreceptor cells,